Protecting magnesium and its alloys



Patented Apr. 2, 1940 UNITED STATES PROTECTING MAGNESIUM AND ITS ALLOYS Jean Frasch, Paris, France, assignor of onehalf to Samuel Fratkine. Paris, France No Drawing. Application February 24, 1937, Se-

rial No. 127,563. In France February 26, 1936 11 Claims.

mineral and organic acids, but also by the salts of said acids, in which magnesium tends to displace the metal nobler than itself.

This is the principal reason why it has not yet been possible to deposit on magnesium, by electrolysis, a nobler metal, for the purpose of pro- 15 tecting same against corrosion; nor have other attempts to protect magnesium, such as oxidation by means of chromates, plating with selenium, arsenic, etc., given any better results, owing to the fact that these deposits, whether formed by displacement or by oxidation, cannot attain suflicient thickness to protect the underlying metal, and also owing to the fact that magnesium oxide, which is appreciably soluble in water, offers little protection against chemical agents, which decompose it and combine with it, thus leaving the bare metal surface exposed to further corrosion.

My invention offers means for efficiently protecting magnesium and its alloys, by a process for vitrifying the surface thereof, which process is 30 preceded by a preliminary operation of anodic oxidation and also (if desired) by mordanting and tinting or painting.

In accordance with my invention, I subject the surface of the metal to anodic oxidation, com- 35 bined with a vitrifying treatment by means of a soluble silicate. I

The anodic oxidation treatment has the effect of coating the surface of the metal with a deposit composed of oxide, hydrate and silicate of magnesium; and furthermore, of creating cavities and/or superficial irregularities in the structure of the metal.

The vitrification is effected through the action of a soluble silicate, such as, for example, NazSiOa, which, in the presence of the oxide and hydrate of magnesium, produces magnesium silicate, which deposits in the form of an adherent and continuous coating containing small crystals of silica liberated by hydrolysis. g fi] Under the influence of exposure to inclement weather conditions, to sea water, etc., said surface coating is gradually transformed into silica which, associated with its supporting layer of magnesia, continues to insure the adequate protection of the metal.

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In the preferred embodiment of my invention, I effect in succession, first the anodic oxidation treatment, and then that of vitrification, by subjecting the metal to the action of two special reagent solutions. However, these two operations may also be combined, by using a single bath containing all the constituents required for both operations. In fact, it is also possible to conduct both operations in succession in a. single bath containing all the constituents for both of said operations, by first passing through the solution the electric current required for the oxidation phase, and then interrupting said electric current to allow the vitrification to proceed.

The essential reagent of the anodic oxidation is a caustic base or a basic salt, preferably in the presence of a soluble silicate. However, I have observed that the results may be improved by adding a small quantity of alumina, for example, in solub.e form; the colloidal alumina liberated is deposited in the minute cavities and irregularities of the metallic surface and improves the adherence and continuity of the protective coating formed in the process of vitrification.

Furthermore, it is advantageous to avoid that said process of vitrification be limited with respect to the extent of the hydrolysis of the vitrifying bath, which might tend to liberate silica in jelly form, namely, at high temperatures, and thus impede the formation of magnesium silicate. For this purpose, to the soluble silicate used in the vitrifying 'bath I add certain substances known as limitors of jelliflcation," such as, for example, caustic bases and alumina, which I prefer to use simultaneously.

The operation of vitrification may be effected with or without electric current, and at moderate or boiling temperature. Furthermore, the two operations of oxidation and vitrification may also be combined with a mordanting process intended to permit coloring of the protective coating. 0n the other hand, a final coat of paint may also be applied to said coating.

In the preferred form of my invention, and by way of example only, I proceed as follows:

(a) For the anodic oxidation of magnesium:

In accordance with my invention, for the oxidation of magnesium I make use of a bath, the essential dissolved constituents of which are a caustic base or a basic salt, to which I add a small quantity of a soluble silicate. The cathodes are of iron. The cell functions either hot or cold, under a tension of 4 to 10 volts, and a current density of 10 to 50 amperes per square foot of anode surface.

The anodic oxidation bath may advantageously have the following composition:

Per cent 1 NaOH 5 to 20 Sodium silicate 2 to 3 Alz(SiO3)3 (precipitated) 2 to 3 KMnO4 0.1 to 0.2

said ingredients being in aqueous solution, and the temperature of the bath being preferably between 40 and 60 C.

I have obtained satisfactory results with an iron or carbon cathode, the anode being formed by the object to be treated; with a tension of 8 volts and a current density at the anode of 50 amperes per square foot, the current flow lasting 15 minutes.

After the above operation, the object under treatment is washed, first in pure running water, then in ammonia water, and finally once more in pure running water.

(b) Mordanting subsequent to anodic oxidation:

This operation is effected in a bath containing the salt of a weak metallic base, such as, for example, aluminium, which, by hydrolysis, deposits its insoluble oxide in the cavities or superficial irregularities formed during the anodic oxidation. This bath functions without electric current and preferably at a temperature materially above that of the atmosphere; it may also contain the dye or coloring matter required to color the protective coating, if such is desired. Otherwise, said dyeing or coloring operations may follow that of mordanting.

In case the alumina is already present in the oxidation bath, the process of mordanting is no longer necessary, and the dye or coloring matter may be applied directly to the object under treatment, immediately after the withdrawal of the latter from the oxidation bath.

The composition of the mordanting bath may be as follows:

Per cent Phosphate of aluminium 15 to 20 Alizarine 3 to 4 Per cent Sodium silicate 5 to 30 NaOH 3 to 4 A12(Sl0s)3' 1 t 2 said ingredients being in aqueous solution, and at a temperature of about 90 C. v

It should be clearly understood that the compositions of the baths given above are only as examples, and are not to be construed as limiting my invention.

The process hereinbefore described affords an efficient and relatively simple means of protecting objects or parts made of magnesium or alloys of the same from corrosion by atmospheric or other agents.

Another advantage of said process is that it adapts itself to the treatment of complete and complicated parts, even when the latter are assembled with aluminium or dural (duralumin) rivets. In this particular case, it enables the -magnesium to be insulated and protected from electro-chemical action, by avoiding the formation of the magnesium-aluminium electric couple.

Finally, the surface coating prepared by the anodic oxidation process or by vitrification constitutes an excellent base for the subsequent application of any paint or coloring matter.

My invention also extends to the novel industrial products which result from the process of vitrification, and also to the different underlying deposits obtained by the process described, as well as to all objects or parts comprising protective coatings or sub-coatings of the type described.

What I claim is:

1. A process for the protection against corrosion. of magnesium and alloys in which magnesium predominates, which comprises subjecting the surface of the same to anodic oxidation in an aqueous bath containing a. caustic alkali in an amount equivalent to to 20% of NaOH, and thereafter subjecting such oxidized surface to a vitrifying treatment with an aqueous solution of a soluble silicate.

2. A process for the protection against corrosion, of magnesium and alloys in which magnesium predominates, which comprises subjecting the surface of the same to anodic oxidation in an aqueous solution containing a caustic alkali in amount equivalent to 5 to 20% of NaOH. and containing a soluble silicate, and thereafter subjecting such oxidized surface to a vitrifylng treatment in an aqueous solution of a soluble silicate, at least one of said aqueous solutions containing a dissolved aluminum compound.

3. A process for the protection against corrosion, of magnesium and alloys in which magnesium predominates, which process comprises subjecting the surface of the same to anodic oxidation in an aqueous bath containing a caustic alkali, a soluble silicate and alumina, and thereafter subjecting the same to a vitrifying treatment with an aqueous solution of a soluble silicate.

4. A process for the protection of magnesium and alloys in which magnesium predominates, which consists in subjecting the surface of the same to anodic oxidation in an aqueous alkaline bath containing caustic alkali equivalent in amount to 5 to 20% of NaOH and containing a soluble silicate, and subjecting such oxidized surface to a vitrifying treatment in an aqueous solution of a soluble silicate which also contains dissolved alumina.

5. A process for the protection of magnesium and alloys in which magnesium predominates, which consists in subjecting the surface of the same to anodic oxidation in an aqueous alkaline bath containing caustic alkali equivalent in amount to 5 to 20% of NaOH and containing a soluble silicate, and subjecting such oxidized surface to a vitrifying treatment in an aqueous solution of a soluble silicate.

6. A process according to claim 3, in which the treatment by anodic oxidation is effected at a temperature between 40 C. and 60 C.

7. A process according to claim 2, in which the anodic oxidation is effected under a voltage of 4 to volts and a current density at the anode of 10 to 50 amperes per square foot, the duration of the treatment being approximately 15 minutes.

8. A process for the protection against corrosion, of magnesium and alloys in which magnesium predominates, which process comprises subjecting the surface of the same to anodic oxidation in an aqueous bath containing from 5 to of sodium hydroxide, 2 to 3% of sodium silicate, 2 to 3% of alumina and 0.1 to 0.2% of potassium permanganate, and following the said anodic oxithe surface of the same dation by a treatment in an aqueous solution of a silicate containing dissolved caustic alkali.

9. A process for the protection against corrosion, of magnesium and alloys in which magnesium predominates, which comprises subjectin the surface of the same to anodic oxidation in an aqueous bath containing as essential reagent a caustic alkali in amount equivalent to 5 to 20% of NaOH, and then to a treatment in an aqueous bath containing 5 to 30% of soluble silicate and 3 to 4% of sodium hydroxide and 1 to 2% of a soluble aluminum compound.

10. A process for the protection against corrosion, of magnesium and alloys in which magnesium predominates, which comprises subjecting to anodic oxidation in an aqueous bath containing from 5 to 20% of alumina and sodium hydroxide, 2 to 3% of sodium silicate, 2 to 3% of alumina and 0.1 to 0.2% of potassium permanganate, and then following the said anodic oxidation by a treatment in an aqueous solution containing 5 to 30% of soluble silicate and 3 to 4% of sodium hydroxide and 1 to 2% of a soluble aluminum compound.

11. A process for the protection against corrosion, of magnesium and alloys in which magnesium predominates, which comprises subjecting the surface of the same to anodic oxidation in an aqueous bath containing 5 to 20% of sodium hydroxide, 2 to 3% of sodium silicate, 2 to 3% of alumina. and 0.1% to 0.2% of potassium permanganate, and thereafter subjecting such oxidized surface to a vitrifying treatment with an aqueous solution of a soluble silicate.

JEAN FRASCH. 

